Dairy farming faces challenges of summer fodder scarcity and methane emissions. To address these issues, one may incorporate plants rich in condensed tannins (CT) into temporary grassland mixtures. However, knowledge regarding the link between CT, milk production and climate remains limited in a field context. We conducted an experiment with four grassland mixtures (grass, grass and legumes, grass and plants rich in essential oils, grass and plants rich in tannins) replicated four times and grazed over six rotations. CT content, dry matter yields, and botanical composition were analysed. We calculated milk production potential (MPP) based on nutrient analysis of the plant material and obtained potential evapotranspiration (PET) data. No differences between mixtures were observed during rotation where PET was low whereas CT content in the tannin mixture was higher in high PET rotation. MPP remained stable over time for the legume, the essential oil and the tannin mixtures but decreased significantly for pure grass. CT content in the tannin mixture was strongly correlated with Lotus corniculatus abundance. Our study suggests that plants produce most CT during peak PET in summer, but forage production is highest during lower PET in spring. Thus, tannin-rich mixtures may primarily mitigate methane emissions in summer.